The present invention relates to a granular powder of melt processable fluorine-containing resin which is suitable as materials for powder coating and roto-molding of powder, and a process for preparing the granular powder.
Fluorine-containing resin powders which are used for powder coating by means of spray coating method, electrostatic spray coating method, fluidized bed coating method or electrostatic fluidized bed coating method and roto-molding or roto-lining are required to have excellent processabilities such as flowability, crushing resistance and film forming property.
In order to give those powder properties to polytetrafluoroethylene (hereinafter referred to as "PTFE") powder, a PTFE primary powder is agglomerated to form a granular powder by utilizing interaction aggregating force between the PTFE particles (U.S. Pat. No. 3,265,679, British Pat. No. 1,100,388, U.S. Pat. No. 3,527,857, U.S. Pat. No. 4,241,137).
However since a melt processable fluorine-containing resin (hereinafter referred to as "MPFR") powder has a lower interaction aggregating force than that of PTFE powder, there could not be employed the above processes for the PTFE powder. Therefore powders of the MPFR for powder coating or the like are prepared according to a process in which a fine powder of the MPFR or its dispersion is thermally treated through spraying it into a high temperature atmosphere (Japanese Tokkyo Kokoku No. 44576/1977), or a process in which a MPFR pellet prepared by a melt extrusion is pulverized.
The former spray drying process, however, is less economical due to requirement of large thermal energy. Also the latter pulverizing process cannot provide a powder product with stable powder properties because powder properties such as particle shape, size and bulk density of the powder product are hardly regulated due to difficulty of control of the pulverization.
As a result of the present inventors' intensive study for providing a MPFR granular powder having stable powder properties, it has been found the fact that an agglomerate of the MPFR particles in which a tetrafluoroethylene polymer (hereinafter referred to as "TFE polymer") is present as a binder has excellent and stable powder properties, and can be prepared by a process similar to the PTFE granular powder preparation processes which have not been applicable to preparation of a MPFR granular powder.